340 EMBRYOLOGY 



At the time of the appearance of the outer segmentation 

 a peculiar change in the position of the embryo occurs. Up 

 to this time its ventral side was greatly curved, for both the 

 anterior and the posterior ends grew toward the dorsal side. 

 Later it assumes the reverse position. This is effected by a 

 rotation of the embryo on its own axis. The movement 

 begins at the anterior and at the posterior parts of the em- 

 bryo, and gradually extends to the middle portion. Where- 

 as the ganglion chain at first lay on the convex side of the 

 embryo, it is now found on its concave surface. In the 

 course of this process the anterior and posterior parts of the 

 body assume their permanent shape (Fig. 157 E). The 

 posterior end is abruptly truncated. A depression on it, 

 which soon makes its appearance, represents the fundament 

 of the sucker. The absence of segmentation at the anterior 

 end is noticeable ; the head, however, is distinct from the 

 anterior part of the body (Fig. 157 E). The mouth-opening 

 makes its appearance as a shallow depression of the ecto- 

 derm far in front, and probably at the place where the 

 medullary gi-oove bifurcated. It unites with the fore-gut, 

 which, as well as the hind-gut, is said to arise from the en- 

 toderm. The entoderm for a long time consists of a compact 

 mass of cells, which increases in length with the growth of 

 the embryo. In the formation of the epithelium, the ceils 

 withdraw to the periphery of the mass ; and the nutritive 

 material, which is surrounded by them, remains at the centre 

 just as in the formation of the intestine in Bhynchelmis. 

 The fore- and hind-guts are the first to be hollowed out. 

 The latter unites with the very short tube which forms the 

 anal invagination located on the dorsal side of the sucker. 

 The entire oesophagus, even the jaws, are said by Salensky 

 to be of entodermal nature ; and only the lips, with their in- 

 ternal lining, are formed of ectoderm. Last of all follows 

 the development of the mid-gut. Even in the hatching em- 

 bryos, which have approximately the development described 

 (Fig. 157 E), the mid-gut is still filled with an undigested 

 yolk-mass. 



To enumerate once more the chief points in the develop- 

 ment of this unique group, which it has hitherto been im- 



